Urethane adhesives are known in the art (Modern Plastics Encyclopedia, McGraw-Hill, New York, N.Y.). Urethane adhesives can provide a very strong bond between surfaces to be joined, and they adhere to a wide variety of surface materials including wood, metal, ceramics, glass, most fabrics, leather and most plastics except high density linear polyethylene, polypropylene and Teflon. In addition, urethane adhesives can be provided to the end user as a single component, a stable, flowable liquid having a stable shelf life but capable of curing within minutes when contacted with water, a hygroscopic material, or humid air. Some examples of moisture-cured urethane adhesives include U.S. Pat. Nos. 5,698,656; 6,657,035; 6,368,714; 5,668,222; and WO 00/44847.
It is well known that most urethanes expand during the curing process because the curing reaction yields CO2 gas which remains entrained within the urethane polymer matrix. Many applications of urethane adhesives are specifically designed to exploit the property of expansion during curing, which is advantageous when surfaces to be joined are rough, irregular or contain gaps or holes that make them difficult to bond.
While useful under certain circumstances, the property of expansion during curing can also be a drawback which has limited the utility of urethane adhesives for applications where it is desired to maintain a thin bond line between the surfaces to be joined. Examples of such applications include woodworking, manufacturing, laminating and the like. For these applications, as well as others, it would be desirable to have a low-expanding urethane adhesive.